In Canada, radioactive wastes are produced since the early '30s, when the first uranium mine began operating at Port Radium in the Northwest Territories. Canada is now the first world producer of uranium (26% of world production), 90% of its production is exported and today there are 20 mines and facilities closed or decommissioned (14 in Ontario, 4 in Saskatchewan and 2 in the Northwest Territories). Radioactive wastes are grouped into three categories: nuclear waste, low and medium activity radioactive waste and waste from the extraction and concentration of uranium and rare earths (Low-Level Radioactive Waste Management Office, 2012). The wastes inventory at the end of 2010 reached 214 million tons from uranium processing and 174 million tonnes of mine wastes. Radioactive wastes generated by uranium processing and mine wastes (uranium and rare earth) require very specific decontamination processes. Atomic Energy of Canada Limited (AECL) is developing a long-term disposal strategy for existing cemented radioactive wastes, which contains significant amounts of uranium, mercury, and a large number of minor elements, including rare earths and fission products. An earlier study indicated that extracting the uranium would be advantageous for decreasing the long-term radioactivity of the waste and, consequently, the cost of the long-term disposal process. Consequently, there are safety and economic incentives for the extraction of metals before subjecting Solid Radioactive Cemented Wastes (SRCW) to a stabilization process. Radioactive elements of uranium and thorium are usually associated with rare earth deposits. The separation of uranium and thorium from rare earths is often a big concern in rare earth industry in order to manage the radioactive nuclides (Zhu et al., 2015). Conversely, uranium ores often contain significant concentration of rare earth. Due to recent increases in both uranium and rare earth prices, there is renewed interest in uranium and rare earth mine sites for developing new ore bodies as well as re-processing the historic waste rock piles and tailings impoundments. Reprocessing Solid Radioactive Mine Wastes (SRMW) may present significant financial and environmental benefits.
The technology for recovering uranium from its most common ores is well established and a vast amount of information is available in the technical literature (Merritt, 1971; Wilkinson, 1962). Uranium is normally leached from its ores with sulfuric acid, separated from impurities using solvent extraction or ion exchange, and precipitated with magnesium or ammonium hydroxide to yield a commercial product, known as “yellow cake”. Extraction of rare earth is also well established. The extractive metallurgy of rare earth from monazite sand, bastnasite ore, and phosphate rock of igneous origin was described by Habashi (2013). This includes mineral beneficiation, leaching methods, fractional crystallisation, ion exchange, solvent extraction, precipitation from solution, and reduction to metals. By contrast, cemented radioactive wastes (SRCW) differ significantly from common ores. SRCW have a unique mineralogy, a high nature, a relatively low U grade, and a high content of Ca (˜35%), SiO2 (˜20%) and Hg (˜1,500 ppm). The chemical composition of mining radioactive wastes (SRMW) could also differ significantly from ores. Some tailings samples are composed of quartz, illite, gypsum, pyrite, microcline, calcite and muscovite. Others are mainly composed of gypsum, quartz, nimite, albite and illite. The composition of the solid radioactive wastes poses significant impediments to the extraction and recovery of metals using conventional technologies. The high Ca content will interfere with both carbonate leaching and sulfuric acid leaching by forming large amounts of CaCO3 and CaSO4, respectively. Furthermore, the high silica content of the cemented radioactive wastes may lead to the formation of colloidal silica, which is known to create severe problems in hydrometallurgical circuits (Queneau and Berthold, 1986). Ion exchange was considered the best method to separate the uranium or rare earth in the leach solution from the impurities and to produce a purified and concentrated solution suitable for yielding a uranium or rare earth products. Most likely, solvent extraction technology cannot be used because of the high concentrations of Al, Fe and colloidal silica, which may cause severe phase separation problems (Queneau and Berthold, 1986; Ritcey and Wong, 1985). The adsorbed uranium and rare earth are usually eluted from the resin with dilute acid or alkaline solutions and subsequently precipitated. The presence of sodium chloride in uranium and rare earth sulfuric leachate is a major problem for nuclear and mining industries. Several researches were done to improve selectivity of resins for metals especially in sodium chloride media.
There is a wide variety of disadvantages and challenges related to the known techniques for treating solid radioactive wastes and metals recovery from radioactive cemented and mine wastes. Main disadvantages are process efficiency and cost-effectiveness. There is indeed a need for a technology that overcomes at least some of the disadvantages of the known methods in the field.